1. Field of the Invention
The present invention relates to a thin film transistor (TFT), a method of fabricating the same, and an organic light emitting diode (OLED) display device including the same, and more particularly, to a thin film transistor (TFT), a method of fabricating the same, and an OLED display device including the same, which can increase the size of a grain in a channel region, effectively protect the channel region of a semiconductor layer at the time of etching process, and reduce processing cost by forming an etch stop layer pattern and crystallizing an amorphous silicon layer using a laser.
2. Description of the Related Art
In general, a polycrystalline silicon layer advantageously has high field effect mobility, can be applied to a high-speed operating circuit, and can be used for configuring a complementary metal oxide semiconductor (CMOS), so that it is usually employed for forming a semiconductor layer of a thin film transistor (TFT). Such a thin film transistor (TFT) using the polycrystalline silicon layer is usually employed for active devices of an active matrix liquid crystal display (AMLCD) and switching devices and drive devices of an active matrix organic light emitting diode (AMOLED).
The thin film transistor (TFT) used for the display device may be classified as either a top-gate type thin film transistor (TFT) or a bottom-gate type thin film transistor (TFT). In the bottom-gate type thin film transistor (TFT), a doped amorphous silicon layer or source and drain electrodes, etc. are formed directly on a semiconductor layer, however, the semiconductor layer may be damaged by an etch margin at the time of etching the doped amorphous silicon layer or the source and drain electrodes, so that the semiconductor layer should be thick. However, when the semiconductor layer becomes thicker, not only its production cost increases but a gate leakage occurs due to the thicker semiconductor layer.
In order to cope with such drawbacks, an etch stop layer pattern may be formed on a region of the semiconductor layer where a channel region is formed. When the etch stop layer pattern is employed, the etch stop layer is usually patterned after the semiconductor layer pattern is formed. In this case, the etch stop layer pattern causes the number of masking processes to be increased by one compared to the conventional process.
In addition, the polycrystalline silicon layer used for forming the semiconductor layer of the bottom-gate type thin film transistor (TFT) may be formed by irradiating an excimer (or exciplex) laser on an amorphous silicon layer to crystallize the amorphous silicon layer. At this time, the excimer laser is irradiated on the amorphous silicon layer to perform crystallization before the etch stop layer pattern is formed. However, it is difficult to maintain a high temperature enough to form a great grain while the amorphous silicon layer is crystallized. In addition, the excimer laser uses a gas source, which makes it difficult to ensure uniformity for every process and requires high costs in equipment management and source to be used.